The project is designed to study the effects of chromosomal aberrations on fetal mortality and development. Comparisons are made between in vivo effects and effects on mouse embryos grown in culture. Some chemical mutagens are known to be capable of inducing chromosomal aberrations such as translocations which may be transmitted to the next generation. Increased rates of fetal death are usually observed among offspring of translocation heterozygotes. It is our purpose to investigate the rates of transmission of such aberrations, and the processes of fetal mortality by direct observation of the development of embryos.